1. Field of the Invention
The present invention relates to an inorganic grouting system for use in a method of anchoring a reinforcing member in a hole, e.g., in a mine roof, wherein reactive inorganic components are introduced into a hole and allowed to react and harden therein around a reinforcing member so as to fix it firmly in the hole.
2. Description of the Prior Art
Anchor bolts are employed in various fields of engineering, for example, as strengthening or reinforcing members in rock formations and in structural bodies. The bolts are inserted into drill holes in the formation or body, and often are fixed or anchored therein, at their inner end or over substantially their entire length, by means of a reactive grouting composition which hardens around the bolt. When used in a mine roof, bolts grouted in this manner help significantly to prevent mine roof failure. Because unsupported rock strata have a tendency to move vertically and laterally, and this motion is what commonly causes the roof to fail, it is important that bolts be installed as soon as possible in a newly exposed roof and that the required strength provided by the hardening of the grouting composition be developed rapidly, e.g., in a matter of a few minutes, or within an hour or so, depending on the type of mine. Rapid hardening also contributes to the efficiency of the bolt installation operation.
As a practical matter, the hardening or setting time of a bolt-grouting composition must be sufficient to allow the reactive components thereof to be mixed and positioned around the bolt in the hole, e.g., at least about 15 seconds, depending on anchoring length, both in the case in which the components are delivered separately into the hole and combined therein and mixed, e.g., by the rotation of the bolt, as well as when the components are delivered into the hole in combined and mixed form either before or after bolt insertion. Beyond this necessary working time, the rate at which the composition approaches its ultimate strength should be as high as possible, e.g., it should attain about 80% of its pull strength in an hour or less, and the ultimate pull strength should be at least about 175 kilograms per centimeter of anchoring length. Thus the over-riding need in grouting systems for rock bolt anchoring is sufficient working time combined with a rapidly attained high ultimate pull strength.
Reactive compositions which have been used in rock bolt anchoring include inorganic cement mortars and hardenable synthetic resins, and these have been introduced into the drill holes through a feed pipe, or in cartridged form. Inorganic grouting systems are economically more attractive than resin-catalyst systems, and generally are not plagued with such problems as instability on storage as are resin-catalyst systems.
Copending U.S. patent application Ser. No. 830,473, filed Sept. 6, 1977, now U.S. Pat. No. 4,126,005, anchoring grouting system and method wherein an inorganic grouting composition has two separate components, one a slush or sludgy mass containing a particulate inorganic cement and a liquid which is nonreactive therewith, and the other containing a liquid which is reactive with the inorganic cement. In use, the two separate components are brought together and mixed, preferably in a hole, e.g., by the rotation of a reinforcing member, and the cement and reactive liquid allowed to react in the hole to form a hardened grout around the reinforcing member therein. The term "cement", as applied in the mentioned copending application, includes cements which set by hydration, e.g., Portland and high-alumina cements; and metal oxides, e.g., magnesium oxide, which set up rapidly when mixed with phosphoric acid or phosphate solutions.
The hardening reaction that occurs when magnesium oxide and phosphates are combined also has been employed for various other purposes, e.g. to produce a binder system for foundry aggregate or refractory materials, to patch or repair cracks in roadways, etc. In these systems the reactants have a low rate of reaction, and are characterized by a long setting time (long pot life or working time) and slow strength development, usually over a period of days. Long pot life allows the mixture of reactive components to be shaped, e.g., by casting, and permits the performance of large jobs with a single mix. For example, U.S. Pat. No. 3,923,534 discloses refractory compositions in which a magnesia of low reactivity (fused or hardburnt magnesia) is used as a setting agent in combination with water and a water-soluble aluminum phosphate binding agent for a refractory filler such as silica or alumina. The wet refractory composition is said to be useful in concrete mixes, as a mortar or grouting, or as a castable composition. Low-reactivity magnesia is used in a minor amount relative to the aluminum phosphate, and the binding agent is a complex phosphate containing aluminum and phosphorus in a 1/1 ratio. These compositions set in hours or even days, allowing large mixes to be used but consequently providing no significant supportive strength over such periods. In addition to lacking early strength, the described compositions develop very little mechanical strength on standing at room temperature even for several days after setting, and require heating, for example, heating in use, to attain a useful mechanical strength.
U.S. Pat. No. 3,923,525 relates to binder compositions for foundry aggregate, the binder system being obtained from an aluminum phosphate containing boron, an alkaline earth material, and water. The composition of the aggregate-binder foundry mix is such as to allow it to be molded or shaped and thereafter cured to form a porous self-supporting structure having good collapsibility and shake-out properties. Only a small amount of binder is used, generally less than about 10 percent, and frequently within the range of about 0.5 to about 7 percent, by weight, based on the weight of the aggregate. Most often, the binder content range by weight is from about 1 to about 5 percent of the aggregate weight. This is sufficient to allow the binder to be distributed on the aggregate particles, and the coated particles to be molded into the desired shape. These foundry mixes require 1 to 4 hours to cure, and the cured shapes are weak enough to be collapsible and readily broken down for removal from a casting.
The method of patching described in U.S. Pat. No. 3,821,006 employs a two-component system of an inert particulate aggregate such as sand and a reactive mixture of an acid phosphate salt and magnesium oxide particles of the "dead-burned" type. Acid phosphate salts disclosed are monoammonium phosphate, monosodium phosphate, and monomagnesium phosphate. None of the disclosed compositions made from these salts have the high early strengths required for rock bolt anchoring in mine roofs. For example, a composition made from monomagnesium phosphate is reported to have developed a compressive strength of only 29 kilograms per square centimeter after 2 hours, and 60 kilograms per square centimeter after 24 hours.
Ammonium phosphate as a binder for magnesium oxide is also described in U.S. Pat. Nos. 3,960,580, 3,879,209, and 3,285,758. The cements based on magnesium oxide and dry, solid monoammonium phosphate (or an aqueous solution of ammonium polyphosphates) of U.S. Pat. No. 3,960,580 contain an oxy-boron compound such as sodium borate to extend their setting time. The compressive strength of these cements even after 2 hours is low, and their maximum strength is not attained for many days. U.S. Pat. No. 3,879,209 describes a process for repairing roadways, etc. with a composition comprising a magnesia aggregate wetted with a solution of ammonium phosphate containing orthophosphates, pyrophosphate, and polyphosphates, the latter including tripolyphosphate and higher polyphosphates. This composition also develops strength slowly, i.e., over a period of days. The ammonium component is described as essential for this composition, as phosphorus oxide components alone, such as phosphorus pentoxide, are disclosed as not giving the desired results. The same ammonium phosphate solution is described in U.S. Pat. No. 3,285,758, which also mentions the unsuitability of phosphoric acid and magnesium phosphate as well.
German OLS No. 2,553,140 describes a process for producing a cement by reacting aqueous orthophosphoric acid with a chemical combination of oxides such as magnesium orthosilicate (2MgO.SiO.sub.2). The cement compositions described have long setting times (9-90 minutes) and their compressive strengths are measured usually after one month.
At the present state of the art, certain oxide/phosphate compositions are known to react extremely rapidly. These contain high-surface-area magnesium oxide, and/or monoammonium phosphate. The reaction with phosphoric acid also has been reported to be extremely rapid. While rapid reaction of the components of a grouting composition for anchoring rock bolts is a desirable property (provided that the composition does not set before it can be mixed and emplaced), it is essential that the composition, as was mentioned above, develop high strength early and attain a high ultimate strength within a reasonable period of time, e.g., in an hour or so, to provide an umbrella of safety in a mine roof. The prior art does not describe or suggest oxide/phosphate grouting compositions that meet these requirements, e.g., compositions that permit sufficient time for emplacing and mixing yet attain a pull strength of at least about 175 kilograms per centimeter of anchoring length in an hour or less.